Poly(trimethylene terephthalate) is produced by reaction of terephthalic acid (TPA) or dimethyl terephthalate (DMT) and excess 1,3-propanediol at elevated temperatures to obtain an esterification product. This esterification product is subjected to a precondensation, and then the precondensation product is subjected to polycondensation to obtain poly(trimethylene terephthalate).
In the poly(trimethylene terephthalate) process, excess 1,3-propanediol is removed by volatilization from the precondensation and polycondensation stages. This volatilized by-product 1,3-propanediol is known to contain several additional by-products, e.g., trimethylene terephthalate cyclic dimer and poly(trimethylene terephthalate) oligomers as well as some carbonyl containing compounds. Furthermore, if the starting material for the process includes dimethyl terephthalate, there may even be small amounts of it found in the by-product 1,3-propanediol. Recycling by-product 1,3-propanediol is desirable in order to improve the efficiency and lower the costs of the process.
Recent experience in operation of continuous processes for producing poly(trimethylene terephthalate), however, has shown that solid by-products in the liquid by-product 1,3-propanediol gradually precipitate on pipes, heat exchanger walls and spray nozzles, etc. The precipitates may cause fouling, which in turn results in lower 1,3-propanediol recirculation flow rates and eventual poor spray condenser operation. This buildup of solids in the recirculation system leads to shortened operational life, increased maintenance frequency and, consequently, higher costs due to increased downtime, maintenance costs and lower overall product yields.
U.S. Pat. No. 6,353,062, U.S. Pat. No. 6,538,076, US 2003-0220465A1 and US2005-0165178 A1 disclose continuous processes for preparing poly(trimethylene terephthalate) by polymerization of bis-3-hydroxypropyl terephthalate. Excess 1,3-propanediol vapors are removed from the process stream and condensed by means of a spray condenser where they are cooled by being sprayed with condensed 1,3-propanediol that has been cooled to less than 60° C., and preferably less than 50° C. The condensed 1,3-propanediol flows into a hotwell where it is combined with additional 1,3-propanediol. A portion of the liquid in the hotwell is pumped through a cooler (i.e., a heat exchanger) to the top of the condenser for use as the condensing spray. None of these documents discloses recycle of excess 1,3-propanediol.
U.S. Pat. No. 6,277,947 and U.S. Pat. No. 6,326,456 disclose processes for producing poly(trimethylene terephthalate) by esterification of terephthalic acid with trimethylene glycol in the presence of a catalytic titanium compound, followed by precondensation and polycondensation. The esterification is effected in at least two stages, where in the first stage a total molar ratio of trimethylene glycol to terephthalic acid of 1.15 to 2.5, a content of titanium of 0 to 40 ppm, a temperature of 240 to 275° C., and a pressure of 1 to 3.5 bar are used. In the at least one subsequent stage, the content of titanium is adjusted to be higher than in the initial stage by 35 to 110 ppm. These two publications disclose recycle of excess 1,3-propanediol into a terephthalic acid/1,3-propanediol paste mixer that is typically unheated. However, the stoichiometry set forth in examples 6, 7 and 8 of both indicates clearly that the recycled 1,3-propanediol did not result from a steady state continuous process. Moreover, the process produced poly(trimethylene terephthalate) with significant color, as suggested by the use of cobalt compounds as color agents in examples 6 and 7.
These problems in recycle of 1,3-propanediol have resulted in reports (see, e.g., U.S. Pat. No. 6,657,044) that it is necessary to remove the solid by-products from the recovered by-product 1,3-propanediol in order to successfully recycle it. U.S. Pat. No. 6,657,044 teaches a process for preparation of poly(trimethylene terephthalate) by esterification of terephthalic acid or dimethyl terephthalate with 1,3-propanediol, where excess 1,3-propanediol is purified before recycle into the process. The 1,3-propanediol stream is boiled and 1,3-propanediol is separated from the high boiling byproduct fraction consisting of solids and semi-solids. The solids and semi-solids are heated in the presence of a metal catalyst which digests and converts the solid by-product to esters of terephthalic acid.
U.S. Pat. No. 6,245,879 discloses procedures for purification of a carbonyl-containing 1,3-propanediol stream for reuse in a poly(trimethylene terephthalate) process.
U.S. Pat. No. 6,703,478 and EP-B1245606 disclose a process for continuously producing an aromatic polyester comprising an aromatic dicarboxylic acid as the main dicarboxylic acid component and at least one glycol selected from the group consisting of ethylene glycol, 1,3-propanediol and 1,4-butanediol as the main glycol component through an esterification or ester exchange reaction and a polycondensation reaction, wherein the distillate containing the above glycol from the polycondensation reaction is subjected to at least flash distillation to remove low boiling substances before recycle to the esterification or ester exchange reaction.
It would be highly advantageous to the continuous poly(trimethylene terephthalate) polymerization process to be able to substantially reduce the amount of fouling due to precipitation of solids from the liquid by-product 1,3-propanediol, particularly in the precondensation stage. In addition, it would be advantageous to be able to recycle liquid by-product 1,3-propanediol into the process with minimal processing, while at the same time obtaining excellent quality poly(trimethylene terephthalate) product.